Method and apparatus for determining the direction of arrival of radio or acoustic signals, and for transmitting directional radio or acoustic signals

ABSTRACT

A directional receiver system may include a receiver, a plurality of receive antenna elements, and a circuit. The receiver may include an input port and an output. The plurality of receive antenna elements may be fixedly configured into a known geometric relationship to each other, and each of the receive antenna elements may be connected to the input port of the receiver. The circuit may be coupled to the output of the receiver, configured to determine time differences at which signals from a source are incident upon the antenna elements, and configured to determine an angular orientation of the source to the receive antenna elements based on the time differences.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application derives the benefit of the filing date of U.S.Provisional Patent Application No. 61/722,644, filed Nov. 5, 2012. Theentire content of this application is herein incorporated by referencein its entirety.

SUMMARY

Systems and methods described herein may include a directional receiveantenna with a plurality of antenna elements connected to a single inputport of a receiver/processor device, the receiver/processor device beingequipped with a process for precisely measuring the time of arrival ofreceived signals and for combining knowledge of the relative positionsof the receive antenna elements and the delays associated with thevarious cables, connectors, pads, or other devices involved, withobserved differences in the time of arrival of signals of interest atpairs of the plurality of antenna elements, to derive accurate angle ofarrival measurements of those signals of interest. In embodiments of thepresent invention in which these times of arrival measurements areaccomplished using digitized signals, the embodiments may resolve thetime of arrival to significantly less than a single sample interval.Embodiments may also include a transmit antenna with a plurality ofantenna elements connected to a single output port of a transmitterdevice, the connections being precisely prepared so that the signalemitted by the transmitter device arrives at the plurality of antennaelements at precisely known relative times, and the physical relativepositions of the antenna elements being rigidly fixed and preciselyknown, thereby enabling a distant receiver, using this knowledge ofgeometry and delays in the transmit antenna, to determine the anglebetween the transmit antenna's orientation and the receive antenna'slocation.

The capabilities of the systems and methods described herein may supporta variety of applications. In addition, the features described hereinmay incorporate transponders, which may be used for example to permitair traffic controllers to determine the range to transponder equippedaircraft.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a representative embodiment of the present invention,comprising a pair of antenna elements, a planar wavefront from a distanttransmitter approaching the antenna elements, and the angle of arrival(AOA) between the line between the two antenna elements and thedirection of the arriving wavefront.

FIG. 2 shows a representative embodiment of the present inventionincluding coaxial cables connecting antenna elements and delay elementand signal combiner to receiver and processor element.

FIG. 3 shows a representative embodiment of the present inventionincluding a planar wavefront moving toward antenna elements.

FIG. 3a is a block diagram of an embodiment of the receiver/processorcircuit 110 of the present invention.

FIG. 4 is a block diagram of an embodiment of a receiver/processorcircuit of the present invention.

FIG. 5 illustrates the effect of re-sampling a digital sample of areceived brief/compressed pulse waveform plus noise, to improve theestimated time of arrival of the peak of the signal, according to anembodiment of the invention.

FIG. 6 shows a representative embodiment of the present inventionincluding receive antenna elements and curve which is the locus ofpossible positions of a signal source whose TDOA between the two antennaelements is a constant value.

FIG. 7 shows a representative embodiment of the present inventionincluding a transmit antenna array comprising of antenna elements, saidantenna elements being held rigidly at a specific distance from eachother by rigid supporting structure.

FIG. 8 shows a representative embodiment of the present inventionincluding a receiver with a single receiver antenna element, locateddistant from the transmitter arrangement shown in FIG. 7, withwavefronts originating from transmitter antenna elements about to arriveat receiver antenna element.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

This detailed description will first refer to an example embodiment of adirectional receiver in which there are two antenna elements 10, 20 (seeFIG. 1), together with other components and processes. Those skilled inthe art will recognize that the arrangements and phenomena describedwill generalize in a straightforward way to other embodiments involvinglarger numbers of antenna elements, and similarly, the arrangements andphenomena described will apply in a straightforward way to transmitterembodiments. Similarly, those skilled in the art will recognize that thearrangements and phenomena described can apply to embodiments utilizingacoustic or RF energy.

Some embodiments of the present invention may use analog implementation.However, this description will focus on embodiments in which digitalsignal processing accomplishes the measurement and estimation functions.

Directional Reception

By connecting a plurality of antenna elements 10, 20 to a single inputport of a receiver processor circuit 110 (see FIG. 3), any errorsintroduced in the electronics of the receiver/processor circuit 110 maybe rendered common mode, so that the effects of these errors cancel outin the calculation of the angle of arrival. Used for reception, the timeof arrival measuring process 215 (see FIG. 3a ) may be able to obtainbetter time difference of arrival precision and accuracy than would bepossible in a system using two separate receivers, other things beingheld equal. The resulting angle of arrival accuracy may make possibleaccurate directional antenna performance with small antenna apertures.

FIG. 1 shows a representative embodiment of the present inventioncomprising a pair of antenna elements 10, 20, and a planar wavefront 40from a distant transmitter approaching the antenna elements 10, 20, andthe angle of arrival (AOA) a 60 between the line between the two antennaelements and the direction 50 of the arriving wavefront 40. Seen in twodimensions, in the plane defined by the location of the distanttransmitter and the locations of the antenna elements 10, 20, if wavefront 40 is effectively planar (so that it cuts the plane formed byantenna elements 10, 20 and the distant transmitter in a straight line),then this angle α 60 is a function of c, TDOA, and b, where c is thespeed of transmission of the wave front 40 in the medium, in meters persecond, TDOA is the time difference in seconds between the time ofarrival of wave front 40 at the nearest antenna element 20 and the timeof arrival at the other antenna element 10, and b is the distance inmeters between the two antenna elements 10, 20. The value of c is knownwith precision; thus if the value of b were precisely known, and theTDOA could be precisely measured, then the angle α 60 could beaccurately calculated.

Those skilled in the art will recognize that the wave front emitted by atransmitter will be approximately planar at long range, and at moremoderate ranges the wave front will exhibit measurable curvature. We usethe planar example for clarity of presentation of the architecture ofthe antenna and its effect on TDOA estimation accuracy.

FIG. 3 shows a representative embodiment of the present inventionincluding the planar wavefront 40 moving toward antenna elements 10, 20.Antenna elements 10, 20 are shown rigidly attached to a rigid structure140 which holds antenna elements 10, 20 at a stable distance from eachother. FIG. 3 also shows broadband signal 150 in the planar wavefront40, about to arrive first at antenna element 20 and later at antennaelement 10. FIG. 3 also shows the time difference of arrival (TDOA) 160of two copies of signal 150 passing through the cable 130, into receiverprocessor circuit 110, which will process the received signal andmeasure the TDOA 160 of the two copies of signal 150. In embodimentsemploying spread spectrum RF signals, after compression or de-spreading,the incoming signal will be effectively a pulse 150, and havingpropagated through the antenna elements 10, 20 and their electricalconnections to the receiver processor circuit 110, pulse 150 will resultin a TDOA 160 observable by receiver processor circuit 110, as describedin FIG. 4 below.

FIG. 3a is a block diagram of an embodiment of the receiver/processorcircuit 110 of the present invention, illustrating a sequence oftransformations that may be applied by receiver/processor circuit 110 tothe energy 170 of signal 50 as collected by the elements of thedirectional antenna and combined by combiner 100 and presented to theanalog section 175 of receiver/processor circuit 110, eventuallyresulting in the output angle of arrival measurement 240. In thisprocess, signal energy 170 is fed to Analog RF Receiver 175 whichproduces the baseband complex signal 180. Signal 180 is presented toconditioner and correlator module 192, which produces matched filteroutput 202. Output 202 is processed by Peak Detection module 204, whichidentifies the correlation peaks 202 and passes them to the TOAestimator 215. The resulting TOA estimates 220 are passed to the AOAEstimator 235. AOA Estimator 235 combines the TOA estimates 220 withknowledge of the antenna geometry and various delays 225 to produce theestimated AOA 240 for each signal of interest.

FIG. 4 is a block diagram of an embodiment of the receiver/processorcircuit 110 of the present invention, illustrating a sequence oftransformations that may be applied by receiver/processor circuit 110 tothe energy 170 of signal 50 as collected by the elements of thedirectional antenna and combined by combiner 100 and presented to theanalog section 175 of receiver/processor circuit 110, eventuallyresulting in the output angle of arrival measurement 240. In thisprocess, signal energy 170 is fed to Analog RF Receiver 175 whichfilters and down-converts analog signal 170 to produce the basebandcomplex signal 180. In some embodiments of the present invention, thisdown-conversion may be to an intermediate frequency (IF) and may resultin a real signal at IF, and the following processes will then operate onthe real signal as appropriate. These implementation choices have noeffect on the desired operation of the present invention, as will beapparent to those skilled in the art. ADC 185 may sample baseband signal180 at at least the Nyquist rate, producing digital signal 190. In thisembodiment of the invention, the signal in use can be a brief pulse, ora signal comprised of a burst of low energy brief pulses which have beenproduced by a possibly sparse spreading code in the transmitter, whichspreading code is known to the receiver, and where thereceiver/processor circuit 110 can be de-spread by use of the spreadingcode, to produce a single compressed pulse having the total energy inthe original pulse burst. Signal conditioner and de-spreader 195 mayapply algorithms to condition the incoming signal 190 and if it was aburst of pulses to de-spread it, producing matched filter output 200.The peak detector 205 identifies the broadband signals of interest 210and passes them to TOA Estimator 215, which may apply algorithms to makeprecision estimates of time of arrival with respect to the local clockfor each signal of interest 210. The peak detection process 205 maycalculate the SNR of each sample of the recovered digital signal andproduce measurements 210 of the arriving signals. The resulting TOAestimates 220 are passed to the AOA Estimator 235, which may combine theTOA estimates 220 with knowledge of the antenna geometry and variousdelays 225 to produce the estimated AOA 240 for each signal of interest.Knowledge 225 may include not only the precise relative positions of thephase centers of the antenna elements 10, 20, but also the precisedelays due to coax cables 70, 90, 120, 130 and delay element 80 andcombiner 100. Those skilled in the art will recognize that the signalsof interest 210 can encode additional information bits, and that theobserved time of arrival (TOA) of the brief/compressed pulse can be usedfor ranging and time transfer applications, so that the presentinvention of precision AOA estimation can fit smoothly within the widelyused architecture that applies digital signal processing algorithms todigital representations of received signals, including various types ofspread spectrum waveforms.

The processor circuit 110 may achieve its increased AOA measuringperformance while receiving and measuring the angle of arrival ofincoming signals from multiple directions without the need for antennasteering, either physical or electrical. For example, if the individualantenna elements are omni-directional, then the directional receiverwill provide AOA measurements from all directions. (Note that thedetermination of angle α 60 may be most accurate when the direction ofarrival 50 is nearly perpendicular to the line between antenna elements10, 20.)

Referring again to FIG. 1, if wavefront 40 is perfectly flat then whenit passes antenna element 20, it will be a distance c*TDOA from antennaelement 10. Elementary geometry shows therefore that the arrival angle α60 will be α=arccos(c*TDOA/b), where b is, as above, the distance, oraperture, between the two antenna elements 10, 20. One skilled in theart will recognize that the AOA will have a left/right ambiguity aboutthe x axis, as shown in FIG. 6.

The accuracy of the measurement of angle 60 may depend sensitively onthe accuracy of the measurement of the TDOA, which in turn is based onthe measurement of the time of arrival (TOA) of wave front 40 at each ofthe antenna elements 10, 20. Errors may be made in the measurement ofthese TOA values, but because the TDOA measurement process and apparatusare so arranged that some of the TOA errors are common mode, thosecommon mode TOA errors will not contribute to the overall TDOA error. Ifthe signals received by the antenna elements 10, 20 can be combined andfed to a single receiver processor circuit 110, then many types oferrors in the processing, such as synchronization of multiple processorsand clocks and instrumentation delay in different devices, will becommon mode, and will not enter into the computation of the TDOA.

Methods of Accurately Estimating Time of Arrival

The first approximation of TOA of such a pulse waveform might be simplythe time of arrival relative to the receiver clock of the sample havingthe maximum amplitude within the pulse. This TOA estimate may havetypical errors on the order of the duration of the sample interval.TDOAs based on such TOA estimates may have unacceptable precision foruse in calculating angle of arrival using antennas of modest aperture.

Considering now the operation of the TOA estimator 215, in oneembodiment of the present invention, using signals 150 comprising eithershort duration pulse waveforms or spread spectrum waveforms which becomeshort duration waveforms after de-spreading, the pulses in signal 150will be of short duration, and basing the TOA estimate on measurement ofthe time of the passage of the peak may be an appropriate approach.Alternatively, one could measure the arrival time by estimation of thearrival time of the leading edge of the signal; this latter approachcould be employed in the case of signals having high band-width, andcorrespondingly sharp rising edges, but relatively long signaldurations. Similarly, this approach could be useful in avoidingmultipath effects in cases in which the multipath delay is longer thanthe rise time of the signal.

FIG. 5 illustrates the effect of re-sampling a digital sample of areceived brief/compressed pulse waveform 150 plus noise, to improve theestimated time of arrival of the peak of the signal, according to anembodiment of the invention. In FIG. 5, the simulated-noise free analogsignal 200 is shown as a continuous line, with simulated noise freesamples 210 of the analog signal shown as asterisks on that line. Addingsimulated noise to the samples with, for example, SNR at the peak of 24dB, FIG. 5 also shows the noisy samples 220 as asterisks on thesimulated continuous signal plus noise 230. In this example, the peaksimulated noisy sample 240 is significantly later than the true peakamplitude of the simulated signal plus noise 250, which itself is veryclose in time to the true peak time 260 of the simulated noise freesimulated signal. Re-sampling at a much higher rate than the originalsampling frequency may thus move the time of the peak sample very closeto the true time of arrival of the peak signal, given adequate SNR ofthe received signal.

The samples 230 in this example are produced by a process known to thoseskilled in the art as “re-sampling” using sinc interpolation. TheNyquist sampling theorem tells us that if the sampling frequency exceedstwice the frequency of the highest frequency component of a real signal,then the real signal can be recovered exactly from the samples. Signals150 that will be band-limited by band-pass filters may be of interest.We may consider the noise present in the received noisy samples 220 asbeing band limited as well, since any out-of-band noise may be filteredout or simply alias into the pass band of our receiver. From the abovewe conclude that our digital signal sample 220 can be re-sampled torecover the continuous signal plus noise 230 to arbitrarily highresolution by using sufficiently high re-sampling frequency. As there-sampling frequency increases, the time of the peak sample in there-sampled data will approach the true peak time of the received bandlimited signal plus noise. This process is illustrated in FIG. 5, wherethe re-sampled (sinc interpolated) signal plus noise peak 250 is closerto the simulated true TOA 260 than the peak noisy sample 240.

In an alternative embodiment, the TOA estimator 215 of FIG. 4 may bedesigned to adjust the sampled signal in such a way that the true peak(or leading edge) arrival time corresponds precisely to the time of thehighest amplitude (or leading edge) sample. One approach is to employthe Delay Theorem of Fourier theory, to use the detected peaks 215represented in the complex frequency domain, and perform a search forthe delay that maximizes the amplitude of the peak sample in eachreceived signal. This may identify the peak arrival time of the signalplus noise, because the peak amplitude is maximum when the time of theparticular sample coincides with the arrival time of the peak. If thesample time is not equal to the peak arrival time, the two samples thatbracket the true peak will have less amplitude than a sample taken atthe peak.

In this process, one may conduct a binary search or other type of searchwithin a limited range of delay values for that additive delay value inthe frequency domain that maximizes the corresponding peak time domainvalue. Since the broadband signals involved may be of short duration,the range of legitimate candidate delay values is small, and the numberof points to be transformed between the frequency and time domains iscorrespondingly small, so that it may be computationally practical tosearch several possible delay values for the desired optimal value. Inthis manner a precise TOA for the signal of interest (plus noise andinterference) may be obtained.

Alternatively, one may match the received signal plus noise with areference copy of the sought signal in either the time domain or thefrequency domain.

For signals 150 whose shape, or shape after dispreading, exhibits a risetime of the order of two nanoseconds or less, even the typical shortdelay multipath encountered may not substantially affect the shape ofthe rising edge of the signal. Therefore one may apply a curve-fittingtechnique addressed to the expected shape of the rising edge of thesignal of interest 150. Taking TOA estimates based on such measurementsof the rising edge, one may obtain accurate TDOA estimates bydifferencing the pair of TOA estimates.

Note that in any case the achievable error depends on theSignal-to-Noise Ratio (SNR), since for low SNR samples the noise canhave a relatively large effect on amplitude of signal plus noise,thereby possibly resulting in a signal plus noise peak distant in timefrom the true peak arrival time of the signal. The possible shifting ofpeak arrival times due to noise is well known to those skilled in theart, and system planning that provides adequate SNR may deal with theproblem.

Method of Calculating AOA—Two Antenna Element Case

The locus of possible transmitter locations (in the plane formed by thetransmitter and receiver antenna elements 10, 20) that correspond to aparticular TDOA δ forms one branch of a hyperbola whose foci are thereceive antenna locations. (The other branch is excluded because theobserved data shows which antenna received the signal first.) Thoseskilled in the art will recognize that by shifting and rotating thecoordinates of receiver antenna elements 10 and 20 such that themidpoint between them is at [0, 0] and elements 10 and 20 are both onthe x axis, one may estimate the arrival angle with the slope of theasymptote of the resulting hyperbola. The slope will be positive ornegative according to which side of the axis of the antenna pair 10, 20the signal arrives from. This is the well known left-right ambiguity ofsuch angle of arrival calculations. For transmitters distant from theantenna array by many times the distance between antenna elements 10 and20, this estimate may be accurate to within a fraction of a degree, theerror approaching zero asymptotically as the range increases.

FIG. 6 shows a representative embodiment of the present inventionincluding receive antenna elements 10, 20 and curve 310 which is thelocus of possible positions of a signal source whose TDOA between thetwo antenna elements 10, 20 is a constant value. Curve 310 is one halfof the hyperbola which is the locus of points whose distances from thepositions of the two antenna elements 10, 20 is the constant c*|TDOA|,where c is the speed of light. Lines 320 are the asymptotes of this halfof the hyperbola. The figure illustrates that for transmitters distantby many multiples of the distance between receiver elements 10, 20, thedirection to the receiver may be closely approximated by the directionof the asymptote.

In FIG. 6, the hyperbola is centered at (0, 0), and receiver antennaelements 10, 20 are located at (−1, 0) and (1, 0) respectively. Asbefore, we may assume the transmitter is nearer antenna element 20 thanantenna element 10, and therefore must be somewhere on the right arm ofthe hyperbola. The difference between the angle of the asymptote and theangle to a point (x, y) on the hyperbola is a function of range, or(x{circumflex over ( )}2+y{circumflex over ( )}2){circumflex over( )}0.5. At ranges of many times the aperture, the locus 310 lies veryclose to the asymptote 320, and arccos(c*TDOA/b) may be a goodapproximation to AOA from (x, y) to (0, 0). At short ranges, the errorcan grow to be significant. Thus for applications in which accurate AOAis required with the range to the transmitter being less than severaltimes the aperture, the arccos(c*TDOA/b), estimate may be insufficientlyaccurate.

The general form of the equation for a hyperbola as shown in FIG. 6 is

$1 = {\left( \frac{x}{u} \right)^{2} - \left( \frac{y}{v} \right)^{2}}$where the slope of the asymptote is m=+/−v/u, the range difference froma point on the hyperbola to the two foci is 2*u, and the distance b/2from (0, 0) to either focus is

$\frac{b}{2} = \sqrt{u^{2} + v^{2}}$

Solving for y in terms of x, we have

$y = {\sqrt{\left( \frac{x}{u} \right)^{2} - 1}v}$Thus for x>>u, the root approaches x/u, y approaches v*x/u and (x, y)approaches (x, v*x/u)=m*x. That is, (x, y) approaches the asymptote.When x is less than several times u, y diverges from m*x and the anglefrom the center to (x, y) diverges from slope m, and the difference is afunction of x. In those cases, one can either apply the above equationusing a value for the transmitter range, if one is known, or use adirectional antenna with a smaller aperture, and correspondingly smallerrange-dependent AOA error.

AOA accuracy will increase with increasing aperture, other things beingheld equal. The impact on AOA error of error in TDOA measurement mayalso decrease with increasing aperture. But AOA error using theasymptote increases with decreasing range, and is significant at shortranges. Therefore in designing systems wherein close range AOA accuracyis required and means of accurately determining range to the transmitterare not available, this tradeoff may need to be evaluated.

Some embodiments of the present invention may enable the receiver toestimate the range to the transmitter by making use of the curvature ofthe incoming signal. One way to accomplish this may be to use threeantenna elements (co-planar with the transmitter, since we are herediscussing the two-dimensional case) on a rigid mounting, with themiddle element closer to one end, and combining the signals from thethree antenna elements as described herein. The three copies of theincoming signal will then present three pairs of signals suitable forAOA estimation as described herein. In applications where the aperturecan be large, this arrangement may give the effect of having both largeand (relatively) small aperture antennas. The asymptote corresponding tothe small aperture antenna pair may more closely approximate the truepath of the signal from the transmitter (to the center of the smallaperture antenna pair), and may be accurate given that the aperture ofthe antenna pair is small compared to the range to the transmitter. Anycurvature in the incident signal will have a larger effect on the AOAmeasured by the antenna pair with larger aperture. For transmitterssufficiently close to the two antenna pairs, these two AOA estimates(adjusted to refer to the same point on their receiving platform) maydiffer, with that from the smaller aperture giving the more accurateAOA, and the difference providing a basis for estimating the range tothe transmitter.

Having thus obtained an estimate to of the range to the antenna, the AOAestimate can be further refined by using the above formula involving xfor points on the hyperbola.

This could be useful, for example, in precision station keeping forlarge aircraft, where a large aperture is available by use of antennaelements on wingtips, and where true ranges from the transmitter in thelead aircraft may be only a modest multiple of the wingtip to wingtipaperture.

Alternatively, in applications requiring accurate AOA at both long rangeand short range one could use two receivers, one using the wide aperturepair for precision long range AOA measurement, and the other using boththe wide aperture pair and a relatively small aperture pair for accurateAOA to close-in transmitters, as described above.

Note that if appropriate, a single antenna element could be arranged toprovide its signal to more than one receiver, using signal splitters,low noise amplifiers (LNAs), pads, and delays as appropriate. Similarly,a receiver configured to switch between antenna feeds could make use ofan antenna array with more than one output line, switching between them,for example to obtain either a large aperture or a small aperture input.Similarly, in a receiver with, for example, two channels, one channelcould take its signal from an array with several antenna elements, toobtain directional information, and the other channel could take asignal from only one antenna element, for example to make recovery of aninformation message easier. There are numerous variations on thisconcept, as will be evident to those skilled in the art.

A single pair of antenna elements may yield best angle resolutionbeam-on and worst at endfire. One way to prevent poor performance insome directions is to use an array of three antenna elements notco-linear with each other. For example, endfire for two of three equallyspaced antennas will provide reasonable performance for the other twopairs, and overall performance may be acceptable in any direction.

Method of Calculating AOA—Multi Antenna Element Case

Those skilled in the art will recognize that the two antenna elementcase applies in the two dimensional domain, as described above.Considering the two element antenna in three dimensions, the transmitterwill be located on a hyperbola of rotation about the axis between thetwo antenna elements. With the addition of a third antenna element, notcollinear with the first two, the geometry will define a threedimensional space containing the plane defined by the locations of thethree antenna elements, with the transmitter not necessarily coplanarwith the antenna elements. That is, the observed AOA will be the anglebetween the ray from the transmitter to the position of the antennaarray on the plane of the three antenna elements. The observed AOA willbe ambiguous as to the side of the plane containing the signal source.Addition of a fourth antenna element, not co-planar with the firstthree, may remove the ambiguity and provide a single three dimensionalAOA. Additional antenna elements can be added, to provide moremeasurements and improved resolution.

Similarly, a plurality of receivers, directional or not, can collaborateto combine their measurements of the received signal to develop AOAestimates, position estimates etc. as appropriate to the geometry of thesituation. For example, a plurality of directional receivers placedseparately at accurately known locations may be configured to observetheir respective AOAs to a particular transmitter, and use the resultinggeometry to localize the transmitter at the best estimate of theintersection of the set of AOAs from the positions of the receivers.

Individual measurements based entirely on directional antennas mayresult in AOA information relative to the geometry of the antenna array.For example, three antenna elements affixed rigidly to an aircraft maysupport AOA with respect to the plane of the three antenna elements. Thecoordinates can be rotated first to the reference axes of the aircraftitself, based on data giving the antenna placement on the aircraftstructure. Then, using information from onboard systems (attitude,compass, inertial navigation system (INS) etc.), the coordinates can berotated from the orientation of the aircraft to the local threedimensional Euclidean space centered at the aircraft and oriented to thelocal horizon. Finally, if global positioning system (GPS) informationis available, or INS is accurate, or AOAs from accurately knownlocations are available, the data can be registered to the GPS grid.

In some geometries the ambiguity (left-right in two dimensions, whichside of the plane in three dimensions using three antenna elements) maybe easily resolved. For example, an aircraft tracking a groundtransmitter will know that the signal comes from below the aircraft, ingeneral. Integrating the receiver AOA information with trackingfunctions may resolve these ambiguities if a series of observationsinvolving moving aircraft or transmitters is available

Another example of a way to obtain unambiguous three dimensional AOAmeasurements is to use an array of four receive antenna elements, withtwo antenna elements combined to feed each of two receiver processorcircuits 110, wherein the axes of the two antenna arrays share a centerpoint and are orthogonal. In such an arrangement, each receiverprocessor element may provide accurate two dimensional AOA measurementin the plane defined by the relative orientation of its pair of receiveantenna elements 10, 20 and the transmitter. By combining the resultsfrom pairs of two dimensional measurements with knowledge of theorientation of the two axes in three dimensions, one can calculate anaccurate three dimensional AOA. For a single transmitted signal, theseAOA observations may have a two fold ambiguity in each of the twoplanes. For guidance of a fast-moving receiver, e.g. an aircraft orother air/space based vehicle, combining two or more observations overeven a short interval may resolve these ambiguities.

Alternatively, one could arrange four antenna elements in a tetrahedron,with three being used to observe AOA with respect to the plane definedby the three antenna elements, and the fourth antenna element pairedwith any of the others providing resolution of the ambiguity. This isonly one of many combinations of directional receiver and geometricconfiguration of the systems and methods described herein that can beadapted for various applications.

Transmission

Employing the principles described herein, a transmitter can be createdthat produces a signal with direction-dependent characteristics. Itshould be understood that this is not directional transmission in thesense of a narrow beam antenna; rather, the transmitter may produce asignal with measurable characteristics (other than amplitude) whichdepend upon the receiver's location with respect to the orientation ofthe transmitter antenna. Such a transmitter could use antenna elementsof any desired radiation pattern, from omni-directional to narrow beam,as appropriate for the application. For clarity of exposition, such asignal with direction-dependent characteristics will be explained byreference to an embodiment containing two transmit antenna elements. Asis the case with the directional receiver embodiment described above,those skilled in the art will recognize that the arrangements andphenomena described may generalize in a straightforward way to otherembodiments involving larger numbers of antenna elements.

FIG. 7 shows a representative embodiment of the present inventionincluding a transmit antenna array comprising antenna elements 410, 420,the antenna elements being held rigidly at a specific distance from eachother by rigid supporting structure 540. Signals from a single outputport of a transmitter 510 move through cable 530 to splitter 500, oneoutput of splitter 500 going via cable 520 to transmit antenna element420, and the other output of splitter 500 going via cable 490 through adelay device 480 and thence via cable 470 to antenna element 410. Thedelays from the transmitter port to each of the transmitter antennaelements 410, 420 are arranged using cables 470, 490, 520 and delayelement 480 to be suitable for the purpose and accurately maintained.The signal from transmitter 510 arrives earlier at antenna element 420than at antenna element 410. FIG. 7 illustrates this time difference byshowing wavefront 430, emitted by antenna element 420, having propagatedfurther than the corresponding wavefront 440 emitted by antenna element410.

The signals from transmitter 510 are emitted from transmit antennaelements 410, 420 with a delay caused by the delay element 480 and thedifference in delays between that caused by cables 470 and 490 versusthat caused by cable 520. The resulting wavefronts 430, 440 incorporatethe net delay imposed by this delay structure.

FIG. 8 shows a representative embodiment of the present inventionincluding a receiver 550 with a single receiver antenna element 560,located distant from the transmitter arrangement shown in FIG. 7, withwavefronts 430, 440, originating from transmitter antenna elements 420and 410 respectively, about to arrive at receiver antenna element 560.For clarity of exposition, the wavefronts are shown as being effectivelyflat, as if the receiver antenna element 560 were far distant from thetransmitter of FIG. 7. As those wavefronts 430, 440 sweep by antennaelement 560, their TDOA 570 as observed by receiver 550 will be the sumof two TDOAs: TDOA 580 and TDOA 590. TDOA 580 is due to the delaystructure of the splitter 500, delay element 480, and cable delays incables 520, 490 and 470 as shown in FIG. 7. This delay may be known toreceiver 550. TDOA 590 is the delay produced by the geometry: the anglefrom the center of the axis between transmitter antenna elements 420,410, and the location of receiver antenna element 560.

Receiver 550 may observe the TDOA 570 of said wavefronts. Given that thestructure and delays of the transmitter apparatus shown in FIG. 7 areknown to receiver 550, the net delay due to cables 470, 490 and 520 plusdelay element 480, shown as delay 580, may be known to receiver 550.Subtracting known net delay 580 from observed TDOA 570, receiver 550 maycalculate effective delay 590. The effective delay may be caused solelyby the geometry of the situation. In the plane defined by the twotransmit antenna elements and the position of the receiver antennaelement, the locus of points defined by the TDOA is one half of ahyperbola whose axis is the line connecting the two transmit antennaelements. Considering the situation in three dimensions, the locus ofpoints defined by the TDOA is one half of the corresponding hyperbola ofrotation.

In FIG. 8, considering a receiver distant from the transmitter apparatusof FIG. 7, wavefronts 430, 440 will pass receiver antenna element 560with a TDOA 580 comprised of the net delay due to the delay structure ofthe transmitter device of FIG. 7, plus the TDOA 590 due to the anglebetween the axis of the transmit antenna structure of FIG. 7 and thedirection from the center of said axis to receive antenna 560.

Upon reception of the signal comprising the two wavefronts 430, 440, thereceiver 550 may process the incoming signal using the techniques andalgorithms described above for processing the incoming signal inreceiver circuit 110 of FIGS. 2, 3, and 4.

Implementation Considerations

The directional reception capability of the systems and methodsdescribed herein may be a basic function, and can be an element of awide variety of systems, some involving little external integration, andothers involving various capabilities in concert to deliver specificsystem capabilities. In this section, we provide some considerations forthe design of directional receiver incorporating aspects describedherein.

Antenna Considerations

Proper functioning of the TOA, TDOA, and AOA processes of thesedirectional receivers may depend on factors relating to the antennas.Those skilled in the art will recognize that the individual antennaelements 10, 20 referred to in this description can themselves bedirectional antenna elements of well known types, such as flat panelantennas, parabolic reflector antennas, yagi antennas, and possiblyothers, which may provide increased gain or other desired properties ina preferred direction or directions, and the use of which will providethat embodiment with increased sensitivity in a preferred generaldirection. The resulting configuration may provide sensitivity in theapproximate direction based on the use of conventional directionalantenna elements, with precision directional sensitivity provided asdescribed herein.

Using methods that are familiar to those skilled in the art, one canbuild a rigid structure to maintain a fixed separation between antennaelements 10, 20, and therefore after careful measurement the distance bbetween antenna elements 10, 20 can be known with precision.

FIG. 2 shows a representative embodiment of the present inventionincluding coaxial cables 70, 90, 120, 130 connecting antenna elements10, 20 and delay element 80 and signal combiner 100 to receiver andprocessor circuit 110. In the example of FIG. 2, the signals fromantenna elements 10, 20 are conveyed to a signal combiner 100, which mayhave the effect of adding the time domain waveforms seen by the antennaelements 10, 20, with delays due to propagation delay in the coaxelements 70, 90, 120 and the additional delay imposed by delay element80. In one embodiment of the present invention, the purpose of thisstructure of delays may be to assure that the net delay seen by receiverprocessor circuit 110 is never less than the sum of delays from coaxconnections 70, 90 and delay element 80, less the delay due to coaxconnection 120. These delays may be planned and implemented such thatthe minimum net delay seen by receiver processor circuit 110 is alwayslarge enough to permit receiver processor circuit 110 to determine thetwo TOA values separately, and identify which signal is associated withwhich antenna element. That is, the delays can be chosen such that thesignals from the wave front 40 originating in the two different antennaelements 10, 20 will never be ambiguous as they arrive at receiverprocessor circuit 110, because for example, by design, waveform energyfrom antenna element 20 will always precede the waveform energy fromantenna element 10 in the combined signal. For waveforms 40 which areeither of sufficiently short duration, or whose duration in theirde-spread/compressed form are sufficiently short, this provision maymitigate risk of destructive interference that might otherwise interferewith operation or reduce SNR. This may prevent the two copies of thesignal from “stepping on” each other, and may remove half of theambiguity in the two-element case. That is, the receiver then knowswhich antenna element originated which signal copy, but the well-knownleft-right ambiguity may remain.

Those skilled in the art will recognize that, using the antenna forreception, signal losses associated with transmission of the signalsdown the coaxial cables and through the combiners may require theaddition of one or more amplifiers and/or pads in order to produce thedesired balance of signal amplitudes arriving at receiver processorcircuit 110, and prevent reflections from corrupting the combinedsignal. Indeed it is normal practice in some embodiments of antennaelements to provide a LNA at the antenna elements themselves, in orderto improve the overall noise figure of the system. These matters arewell understood and those skilled in the art will see from the presentdrawings and descriptions how such modification would be made to createvarious embodiments of the present invention and will recognize thatsuch modifications could easily be made in such a way as to preserve thedesired effect of the present invention.

Additional amplifiers and pads may be employed to reduce reflections andachieve desired relative signal levels. That is, low noise amplifiers,pads, and combiners to combine the signals from the plurality of receiveantenna elements may be employed so that the combined signal at thereceiver is free of reflections and the relative amplitudes of theindividual signals are adjusted to the desired relative levels.

By use of suitable RF splitters, LNAs, pads etc., antenna elements maybe configured to provide signals to more than one receiver according tothe present invention. Thus for example a plurality of receiver antennaelements in a suitable geometry could be configured to provide pairwisecombinations of signals to a plurality of receiver circuits 110according to the present invention, with the TDOA data from theplurality of receivers being combined to provide unambiguous threedimensional AOA measurements. Such an arrangement may minimize theself-interference effects that would occur from having more than twosignals combined on a single receiver input, at the cost of employingmultiple directional receiver circuits 110. Similarly, it might bedesirable to have both a directional receiver according to the presentinvention and a conventional receiver for other purposes, such asrecovery of an information message, or processing for different signalsor using different algorithms. In such a case, one of the antennaelements used by the directional receiver could be arranged by the abovesplitter/LNA etc. method to provide a single-antenna signal for thesecond receiver.

Self-Interference, and Methods for Minimizing Self Interference

One effect of using a plurality of receiving antennas, all connected toa single input port of a receiver processor circuit 110, is thatmultiple copies of the transmitted signal 150 may be delayed and addedtogether into the single real analog signal 170. Signals 150 may oftenhave durations varying up to hundreds of microseconds or longer, makingit impractical to insert delays in the input connections of sufficientduration to assure that the energies from input antenna elements 10, 20do not overlap in time. Therefore, for much of their durations thesignal energies from signal 150 collected separately by antenna elements10, 20 may overlap in time, resulting in constructive or destructiveself interference at those times when signal energies from both copiesof signal 150 are present. This self-interference may result in a lossof SNR for each of the signals recovered in the recovered transmittedsignal 200. Use of additional antenna elements connected to a singleinput port of receiver processor circuit 110 may exacerbate thisproblem.

The above problem may be minimized by limiting the number of receiverantenna elements to two. If the signal of interest is a sufficientlybrief single pulse, this problem can also be alleviated using suitabledelays 80 to assure that duplicate copies of the pulse do not overlapeach other in the composite signal 130. Moreover, by using sparsespreading codes, the problem may be reduced further. Indeed, thesparseness, or duty cycle, of the spreading code can be set to result ina wide range of reductions of this problem of self-interference.

Processing Requirements, Synchronization

Some embodiments of the present invention can obtain the desiredbenefits with the use of modest processing capabilities. This can beimportant in size/weight/power effects. For example, full real timeprocessing capability can require collecting and sampling a giga-sampleor more per second, with digital signal processing (DSP) steps in boththe time and frequency domains, entailing hundreds or a few thousanddigital Fourier transformation operations on 2{circumflex over ( )}20complex samples. Even with current processing capabilities (large FPGAs,advanced graphic processing units (GPUs)) such sampling and processingcan require resources that may be prohibitive for small, perhapshand-held or man-carried applications. These requirements can besubstantially reduced in applications in which there is a meansavailable to the receiver to determine the time of arrival of thesignal, to reasonable approximation.

For example, in applications wherein the GPS is available, at least tothe extent of reliable pulse-per-second (PPS) output from the GPSdevice, the transmitter and receiver can collaborate on a plannedschedule of transmissions. Such synchronization may not need to beaccurate to the nanosecond; reduction from full real time collection toa few milliseconds per cycle can make enough difference insize/weight/power (SWAP) characteristics.

Depending on the requirements of the particular application,synchronization can be accomplished by GPS, by stable or accurateinternal time source(s), by use of a common triggering signal, or byother means. Similarly, in an embodiment using the transponder mode, inwhich the receiver interrogates the transmitter, which responds with aknown time delay, the receiver may need only listen for a time intervallong enough to hold the signal, plus an increment for range uncertainty,plus margin for any other uncertainties. For an RF system, theseuncertainties can be small—light travels a mile in about 6microseconds—so the required reception window may be small enough thatan ordinary processor could easily manage it—no FPGA or GPU required.

In some applications of the present invention, the same transmittersignal can be used by both long range receivers, to assist in localizingthe transmitter, and by short range, hand-held receivers. An examplewould be a rescue system in which a person in distress activates atransmitter, and the signal is received aboard a rescue helicopter,which then proceeds to the vicinity of the transmitter. If thetransmitter is in a building, the rescuers may use a small handheldreceiver that responds to the same signal, in order to proceed to theright location in the building. Such a hand-held device could trigger onthe signal with only modest processing, due to the strength of thesignal being higher at short range. That is, the system may be sodesigned that reception at long range would require the processing gainobtained by de-spreading, but at close range the transmitted signalitself could trigger the receiver, so that a close range hand-heldreceiver could collect the signal over a short interval.

Similarly, where battery size is a concern, in applications in which mayneed to be in a “seek” mode for only a brief time, full scale real timeprocessing may be accomplished with modest battery drain if the seektime is short. Applications with this property may include for example agroup identification approach in which members of the group are equippedwith transmitters, and a portable battery operated receiver may needonly to collect the signals on an occasional basis. Such a portablereceiver system might be employed, for example, to search for hikers whohave become lost in the wilderness

Waveform Considerations

In this section we will focus on RF embodiments, and more narrowly onaspects of broadband waveforms in relation to embodiments of the presentinvention. Broadband waveforms may be of particular interest becausethey typically have fast rise-times and/or sharp peaks, which makes themsuitable for the precision time of arrival measurements that enable goodangle or arrival measurements.

Those skilled in the art will recognize that various broadband waveformsare available for which the systems and methods described herein mayoperate efficiently, and that these varieties may include some whichhave additional capabilities, such as data transmission, jam resistance,and low probability of detection (LPD). Accurate AOA determination maybe added with good performance, using antenna arrays of modest aperture,to components of existing systems, resulting in valuable improvement insystem effectiveness, as shown by brief discussions of some examplesbelow.

In one embodiment, the RF signal of interest 150 being transmitted bythe distant transmitter may be a short pulse, or equivalently any of awide selection of spread spectrum signals, which may be de-spread by thereceiver processor circuit 110, resulting in a short baseband pulse. Inone class of embodiments such a spread spectrum signal may be comprisedof a sparse, low duty cycle burst of short pulses. Use of sparse, lowduty cycle spreading codes may provide several advantages, includingreduction of negative effects due to destructive self-interferencesupport of ling codes, support of many different codes (as compared toconventional orthogonal CDMA codes, of which there are not largenumbers), and potential desirable LPD and anti jam characteristics.

Transmitters may use signals and waveforms having low probability ofdetection characteristics, for example by employing sparse widebandspreading codes and other waveform characteristics known in the art andused to make signaling systems secure.

Some embodiments may involve information messages as well as individual(compressed) pulses. Such messages, involving repetitions of thespreading code, may provide large numbers of observations which can beused to both smooth the history of AOAs and, for moving participants,provide data for localizing and tracking.

In embodiments using spread spectrum signals to transmit information,the signal of interest 150 may be a series of short pulses resultingfrom application of a spreading code to an information signal. Moreover,in some embodiments there may be a plurality of spreading codes in use,allowing a plurality of users to operate in a particular band, usingstandard CDMA (code division multi-access) technology. A variety ofspread spectrum CDMA systems are standard art; the global positioningsystem (GPS) is one example. In such embodiments, the de-spread signal200 may contain a corresponding series of baseband pulses for each ofthe plurality of receive antenna elements. In such an embodiment, peakdetector 205 may recover the series of peaks corresponding to each ofthe plurality of receive antenna elements, and TOA estimator 215 mayassociate those peaks originating from a particular receive antennaelement with that particular receive antenna element. The sequencecorresponding to a single receive antenna element may provide receiverprocessor circuit 110 with the information signal originally sent by thedistant transmitter. In such an embodiment, AOA estimator 235 may obtaina corresponding sequence of AOA estimations, each corresponding to asingle de-spread peak in the original spread spectrum signal 150, andeach being associated with its TOA. Such a sequence of AOA estimations240 may provide a history of AOA values over time. Such a history overtime may be useful in localizing or tracking a transmitter from a movingreceiver, or in tracking a moving transmitter or in validating thesource of the signal (for example detecting spoofing attempt based oncomparing observed AOA to expected AOA from an authorized transmitter).

Transmitters that employ occasional transmissions, as may be appropriatefor tagging and tracking rather than general communication purposes, canbe configured to encode ID information and optionally additionalinformation bits to convey status or other similar data. These signalcharacteristics can be compatible with AOA measurement in accordancewith the present invention.

For spread spectrum signals, use of sparse codes may reduce potential ofself-interference. That is, spread spectrum signals, with or withoutfrequency hopping, may be employed with chips of duration short enoughthat practical delay elements in the receiver antenna structure canassure that the same chips in the incoming signal do not step on eachother in the combined signal. Sparse spreading codes in the signal mayreduce the likelihood of any two chips in the incoming signal, even indifferent locations in the spreading code, stepping on each other in thecombined signal, thereby reducing the effect of self-interference.

Low duty cycle signals, in the sense that the spreading codes are notrepeated continuously as is the practice in conventional spread spectrumapplications like GPS, but rather are transmitted individually, in someembodiments using preplanned precision time intervals between saidtransmissions of a single spreading code, may reduce difficulties due toself interference, while timing information and information bits can beencoded in the series of transmitted spreading codes.

Transponder Mode

In one class of embodiments, the transmitters and receivers may be setup to operate in transponder mode. That is, the directional receiversmay be configured with a suitable RF interrogator, and the transmittersmay be configured with a capability to receive the interrogation signaland to respond to the interrogation with a precisely determined delay.Such a transmitter is referred to as a transponder. In such a system,the transponder delay may be known to the directional receiver, therebyenabling the directional receiver device to obtain both accurate AOAinformation as before, plus ranging information accurate to as little asa fraction of a meter, based on knowledge of the flight times and delaysof the signals involved, all without benefit of GPS. One embodiment maycombine the interrogator and AOA receiver in the same device, so thatthe plurality of interrogator/receivers, and the plurality oftransmitter/responders, would comprise a transponder system.

In some embodiments it may be preferred not to include the transpondercomponent. In some applications inclusion of the transponder componentmay materially improve performance of the overall system, because theaddition of accurate range information enables the use of well knowntechniques of multilateration in estimating the location of thetransmitter. Indeed, the performance of multilateration systems can besignificantly improved by the addition of angle of arrival informationper the present invention.

For example, some existing multilateration systems are enabled tooperate without GPS at the transmitters by use of highly stable clocksin the transmitters, so that the receivers, using receptions atdifferent times and locations, can localize by solving for transmitterclock drift as well as position. This is equivalent to what GPS does, insolving for latitude, longitude, elevation, and receiver clock error.For this to work, the transmitter must have a stable time source thatexhibits stable drift, so that drift can be accurately characterized bya single parameter over the appropriate time intervals, and theresulting calculation has only four unknowns. Many observations arerequired, and localization accuracy is limited, with typical errorsseveral times the typical GPS localization error.

Adding AOA information may remove ambiguities and possible pathologicalcases that may otherwise be present in multilateration calculations, andmay provide an additional independent measurement, which may reduceexpected errors in the solution. Moreover, reliance on AOA observationscan relax the criteria for transmitter clock stability, making thetechnology usable with a wider variety of transmitters.

With the addition of transponder mode, the system may obtain accuraterange information. Other multilateration systems operate with what areknown as pseudo-ranges—that is, direct measures of the time of flight ofa signal, where the dominant error is clock drift. Receivers configuredusing the techniques described herein and including transponder mode mayobtain precise ranges, not pseudo-ranges, which may reduce thedimensionality of the calculation and remove clock drift as a factor.

Those skilled in the art will recognize, in many of the exampleapplications described below, that the inclusion of transponder mode maymake major improvements in system performance.

Example Applications of the Present Invention

To illustrate the broad range of applications of the present invention,we describe several examples. These are examples only, and those skilledin the relevant arts will realize that many other variations arepossible.

Detection/Correction of GPS, INS Drift and Errors

For example, in one embodiment, GPS receive antennas and processor couldbe configured as described herein in order to provide the receiver withaccurate AOA estimates over time for the satellite or pseudo-satellitesignals being used in the GPS solution, thereby making possible thedetection of spoofing attempts by hostile transmitters of “GPS” signalsby detecting that they are coming from the wrong direction. Thedirectional accuracy obtained by small aperture receive antennas mayimprove the effectiveness of the approach to GPS spoofing detectionusing AOA measurements, since many GPS installations lack room forantennas of more than modest aperture.

Similarly, absent GPS, long range or long duration flight navigation byINS is subject to INS drift, and use of accurately pre-positioned groundtransmitters could provide corrections; this function may beparticularly effective in transponder mode.

Airborne Applications

The capabilities offered by airborne embodiments of the presentinvention can support numerous valuable applications, depending on thenature and mission of the aircraft or other air/space based vehicleinvolved, the transmitters to be used (tags intended for this use, orexisting transmitters with other purposes, etc.) and other factors suchas whether the transmitters are stationary or mobile, what is known apriori about the transmitters, etc.

A Plurality of Fixed Ground-Based Receivers Track Aircraft

Each of a plurality of receivers may obtain a directional reception froma particular transmitter. Data (including AOA measurements) from thereceivers, together with knowledge of the locations of the plurality ofreceivers, may permit three dimensional localization calculation for thetransmitter of the received signal. So, for example, an aircraftequipped with a transmitter could be localized by a fixed set ofdirectional receivers, for example placed near an airport. Such a systemcould fill the function of radar for tracking such aircraft.

Aircraft Equipped with AOA Receivers Localize and Track Transmitters

In one example embodiment, an aircraft with AOA receiver obtains AOAobservations for airborne or ground transmitters, and may be able tolocalize and track those transmitters. For simplicity of exposition, wedescribe the case where the aircraft has three antenna elements rigidlyfixed to its structure in a triangular arrangement, for example bothwingtips and near the tail. Note that this wide aperture may providegood AOA accuracy. If flexing of the aircraft structure becomes aproblem, the calculations could be calibrated using accelerometers toadjust the geometry of the antenna elements appropriately.

One embodiment of the present invention would be a receiver on board anaircraft, with three receive antenna elements fitted rigidly to the airvehicle in geometry suitable to support three-dimensional angle ofarrival observation. Incoming signals may result in accurate threedimensional AOA measurements in accordance with the previously describedsystems and methods. These AOA measurements may provide the anglerelative to the plane containing the three antenna elements, and thatplane would provide a fixed and known geometric relationship for theantenna elements on the aircraft. (Because aircraft structure can changeslightly with wing loading, it may be appropriate to include acalibration calculation based on accelerometer or other instrumentreadings to adjust the exact geometry of the antenna array to compensatefor these changes in aircraft structure in flight.) There may be theambiguity as to which side of the plane the signal was arriving from,but as previously discussed, this ambiguity may be resolved based onknown geometry of the situation and/or analysis of measurements fromdifferent receiver positions.

Internal to the receiver, the AOA may be calculated relative to theplane defined by the receive antenna elements and transformed to theaxes of the aircraft. To use these values for navigation purposes, or toassist in localizing the transmitter, the aircraft system can useinformation from flight instruments and/or external aids such as GPS toconvert the angles with respect to the plane of the receive antennaelements into the angles with respect to the frame of reference withinwhich the aircraft is operating, for example the GPS Earthcentered—Earth fixed (ECEF) frame of reference. If no accuratepositioning data, such as GPS, is available, one could still obtain 3DAOA with respect to the local horizontal, and the aircraft could stillbe flown directly to the transmitter using well known DF (directionfinder) flight techniques.

In one localization application, an aircraft equipped with a threedimensional AOA receiver can, by observing accurate 3D AOA of a fixed orslow moving transmitter on the ground, from each of a series ofpositions along its flight path, where onboard instrumentation such asINS or other means provides the aircraft with accurate flight path data,obtain accurate relative location (subject to GDOP—geometric dilution ofprecision—as used in GPS applies to the present invention as well)information for the ground transmitter. If the aircraft is able toregister itself to a grid such as the GPS frame of reference, then theobserved localization of the transmitter will also be registered to thatgrid.

The transmitter may be moving slowly compared to the velocity of anairborne receiver, and the localization system may obtain an estimate ofthe transmitter's trajectory from a sequence of AOA observations,optionally augmented with terrain data if the moving transmitter isground-based.

Guidance of Aircraft to Ground Transmitter

In another application, by using a three dimensional directionalreceiver embodiment of the present invention, an aircraft can use thereceived three dimensional AOA information to guide its flight to thelocation of the transmitter. In such an application, the receiver couldbe incorporated in an aircraft, and the transmitter could be attached toa destination (landing zone, for example). For a stationary target, sucha guidance capability would permit the aircraft to fly directly to thetransmitter with no other external guidance information required. For amoving target, provided the target motion were slow compared to thespeed of the aircraft, and provided the target motion did not exceed thedynamic motion capability of the aircraft, the aircraft could still flydirectly to the target.

In a similar application, the receiving system could be used, forexample, on a rescue helicopter with the transmitter being operated bypersons in need of rescue or extraction. A helicopter pilot could usestandard piloting techniques to fly a DF approach to the transmitter.Data from the receiver could be used to create a software instrumentdisplaying standard aircraft instrument images to the pilot. Thisdisplay could also use data from onboard instruments, digital maps, etc.to improve the presentation.

Blue Force Tracking

The localization capabilities inherent in the present invention can beapplied to blue force tracking. For example, a standoff surveillanceaircraft with directional receiver could track three dimensional AOAsfor all transmitters in use by dismounted individuals, ground vehicles,ground installations, and/or other aircraft in the coverage area.

Even a single aircraft can equipped with an omni-directional antennaarray can observe, identify, and obtain AOA estimates from each suitabletransmitter in line of sight. This AOA data could be combined withonboard navigation and map data to estimate transmitter positions.

A plurality of receiver-equipped aircraft, if they have good positioninformation, can collaborate to obtain position estimates for atransmitter in view. The intersection of their azimuth and elevationobservations may provide a reasonable position estimate.

If the receivers on such aircraft are operating in transponder mode,where a receiver aircraft sends an interrogation to the transmitter andthe transmitter responds, the interrogator may obtain accurate range, aswell as azimuth and elevation. Given the receiver's good positioninformation, these range, azimuth, and elevation observations can beregistered to the ECEF (earth centered, earth fixed) grid.

If there are a plurality of receivers collaborating in transponder mode,a concern might arise that too many interrogations and responses wouldbe required. This problem can be addressed as follows: One interrogationis sent, say by receiver RX1, and is heard by receiver RX2, as well asby the transmitter. The transmitter sends its response, as usual. RX2also hears the response. Then by combining observations of RX1 and RX2,the system can obtain the sides of the triangle formed by positions ofRX1, RX2 and the transmitter, in the following manner: RX1 knows thetime it sent the interrogation, and the time the response was received.This difference will be just twice the flight time of the signal, plusthe programmed delay of the transponder. So RX1 can calculate r1, therange to the transmitter, by observing the TOA of the response.Similarly, RX2 can use its TOA of the transmitter's response tocalculate the sum of r1 plus r2, the distance from the transmitter toTX2, less r12, the distance from RX1 to RX2. So combining the data, wehave:

-   -   1. RX1: notes the time it sends the interrogation    -   2. RX1: calculates r1, observed from the transponder reply        signal from the transmitter    -   3. RX2: observes the time the interrogation arrives, and from        the time of transmission recorded by RX1, calculates r12, the        range from RX1 to RX2.    -   4. RX2: observes the TOA of the transponder response sent by the        transmitter, and from that, and knowing the time RX1 sent the        interrogation, calculates r1 plus r2, obtaining r2.

Thus the two receivers and the transmitter in collaboration can obtainthe sides of the triangle formed by the locations of the two receiversand the transmitter, and therefore solve the triangle. Assuming theparticipants are aircraft, addition of compass data can orient thetriangle. If multiple receivers are collaborating, there are moretriangles that can be solved and oriented in similar ways, and combinedto produce relative positioning and orientation of all participants.Errors can be reduced using methods familiar to surveyors who mustestimate three-dimensional positions based on numerous observations.Note also that registering such two dimensional triangles in threedimensions can be aided by the azimuth and elevation observations of thereceivers involved.

In this fashion a plurality of receivers can accurately localize aplurality of transponders by use of a single interrogation, assuming allparticipants are in line of sight range of each other.

Note further that even if GPS is unavailable and accurate position isnot known by the receivers a-priori, these measurements by thereceivers, both azimuth/elevation and range estimates, do not dependupon receivers' knowledge of its their positions. The errors in theresulting triangle(s) side measurements may depend on the stability ofthe receiver clocks, and may be accurate to the resolution of the TOAobservations (plus very small uncertainty in transponder delay). Thus itis not unreasonable to expect the triangle dimensions to be highlyaccurate, depending on the quality of the implementation.

Note further that the above arrangement has the virtue that only one (ora few, in case there are many participants and over an area thatincludes line of sight restrictions) interrogation may need to be sentfor multiple receiver and transmitter units in range of each other. Thusthe number of responses transmitted may grow only linearly with thenumber of transponders, and may be independent of the number ofreceivers.

Aircraft Positioning from Surveyed Ground Transmitters

In one class of applications, the transmitters can be fixed at preciselysurveyed locations, and can include their ID in their transmissions, anddata regarding the positions and IDs and transmission schedules of thetransmitters can be provided to the receiving systems.

In such a configuration, the aircraft can obtain navigation informationeven without access to GPS. If the ground transmitters are accuratelysynchronized (so that their relative transmission times are known towithin nanoseconds), the receiver on the aircraft can obtain positionfrom comparison of the TDOAs of different transmitters, using well knownmultilateration techniques such as are used for positioning in LORAN.This multilateration plus accurate AOAs provided by the systems andmethods described herein may provide accurate real time localization,without reliance on GPS if the synchronization means does not use GPS.Alternatively, the ground transmitters can be synchronized by somemeans, such as a local timing signal, accurate onboard clocks, or othermethods. Note that this synchronization may not necessarily depend theprecise time; the exact time of transmission may be unnecessary, becausemultilateration relies only on the relative timing. Non-GPSsynchronization could be obtained, for example, by timing pulsestransmitted by wire, or by RF.

If the above accurately positioned ground transmitters are notaccurately synchronized, the aircraft with the directional receiver candevelop accurate track information by integrating three dimensional AOAto each transmitter taken at a plurality of times along the aircrafttrack; these data can be augmented by track information obtained fromonboard instrumentation such as INS to provide accurate positioning withrespect to the ground transmitters. Assuming the transmitters arepositioned reference the GPS grid, the resulting localization and trackinformation in the aircraft may also reference the GPS grid. And if theembodiment includes transponder mode, the measurements may be much moreaccurate.

Air to Air Applications

The present invention may be applied with both transmitter(s) andreceiver(s) employed on aircraft, for example, in precision stationkeeping for aircraft flying in formation, where a large aperture isavailable by use of antenna elements on wingtips, giving accurate AOA atlong range, to support rendezvous. Where true ranges from thetransmitter in the lead aircraft may be only a modest multiple of thewingtip to wingtip aperture, it may be appropriate to use a smalleraperture antenna, or to provide both large and smaller aperturearrangements.

An example of such an instance would be an aircraft approaching anairborne tanker for refueling. This application may require both longrange operation, for rendezvous, and precision short range operation formaintaining connection during the fueling operation. If the approachingaircraft had directional receive antenna elements on its wingtips, andrequired precise AOA from a transmitter mounted on the refueling drogueof the tanker, close range AOA could be obtained using a receiveraccording to the present invention having an appropriately smalleraperture.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example and notlimitation. It will be apparent to persons skilled in the relevantart(s) that various changes in form and detail can be made thereinwithout departing from the spirit and scope. In fact, after reading theabove description, it will be apparent to one skilled in the relevantart(s) how to implement alternative embodiments. Thus, the scope of theinvention should not be limited by any of the above-describedembodiments.

In addition, it should be understood that any figures which highlightthe functionality and advantages are presented for example purposesonly. The disclosed methodology and systems are each sufficientlyflexible and configurable such that they may be utilized in ways otherthan that shown.

Although the term “at least one” may often be used in the specification,claims and drawings, the terms “a”, “an”, “the”, “said”, etc. alsosignify “at least one” or “the at least one” in the specification,claims and drawings.

Finally, it is the applicant's intent that only claims that include theexpress language “means for” or “step for” be interpreted under 35U.S.C. 112, paragraph 6. Claims that do not expressly include the phrase“means for” or “step for” are not to be interpreted under 35 U.S.C. 112,paragraph 6.

I claim:
 1. A directional receiver system comprising: a receivercomprising at least a single input port and an output; a plurality ofreceive antenna elements configured into a known geometric relationshipto each other, each of the receive antenna elements being connected tothe single input port of the receiver by one of a plurality of circuits,each of the plurality of circuits adding one of a plurality of known,fixed delays such that outputs from each of the receive antenna elementsare received at the single input port of the receiver; and a circuit,coupled to the output of the receiver, including a time of arrivalestimator configured to determine, from the outputs received at thesingle input port of the receiver, time differences at which signalsfrom a source are incident upon the antenna elements and an angle ofarrival estimator configured to determine an angular orientation of thesource to the receive antenna elements based on the time differences andthe known, fixed delays.
 2. The system of claim 1, wherein the pluralityof receive antenna elements comprise at least one antenna element pairwith a first aperture and at least one antenna element pair with asecond aperture smaller than the first aperture.
 3. The system of claim2, wherein the angular orientation is determined based on a differencebetween signals received by the at least one antenna element pair withthe first aperture and signals received by the at least one antennaelement pair with the second aperture.
 4. The system of claim 1, whereinthe plurality of receive antenna elements comprises three receiveantenna elements, and the determined angular orientation is atwo-dimensional orientation or a three-dimensional orientation.
 5. Thesystem of claim 1, wherein the plurality of receive antenna elementscomprises at least four receive antenna elements, and the determinedangular orientation is a three-dimensional orientation.
 6. The system ofclaim 5, wherein the angular orientation is determined based on acombination of angular orientations obtained by pairs of receive antennaelements.
 7. The system of claim 1, wherein the receiver is mounted on avehicle.
 8. The system of claim 7, wherein the vehicle is an aircraft.9. The system of claim 8, wherein the circuit is further configured toreceive a signal indicating a change in the known geometric relationshipand compensate based on the change when determining the timedifferences.
 10. A system comprising a plurality of the systems ofclaim
 1. 11. The system of claim 10, wherein: the plurality of receiveantenna elements comprises at least one antenna element pair with afirst aperture and at least one antenna element pair with a secondaperture smaller than the first aperture; and the angle of arrivalestimator is further configured to estimate a range to the source bycombining the signals received by the at least one antenna element pairwith the first aperture and the at least one antenna element pair withthe second aperture.
 12. The system of claim 1, wherein: the receiver isa single receiver; and signals from each of the receive antenna elementsare combined and fed into the single input port of the single receiver.13. The system of claim 1, wherein: the plurality of receive antennaelements comprises at least two independent antenna arrays, eachindependent antenna array comprising one or more receive antennaelements; and the receiver is further configured to switch inputs to thesingle input port so that it selectively receives signals from theindependent antenna arrays.
 14. The system of claim 1, wherein: thereceiver is one of a plurality of receivers, each of the plurality ofreceivers comprising at least a single input port and an output; theplurality of receive antenna elements is one set of a plurality of setsof receive antenna elements, each set of receive antenna elements beingconnected to the single input port of one of the plurality of receivers;the circuit is one of a plurality of circuits, each circuit beingcoupled to the output of one of the plurality of receivers, each circuitincluding the time of arrival estimator and the angle of arrivalestimator; and the time of arrival estimator and the angle of arrivalestimator of at least one of the plurality of circuits are configured todetermine a distance between at least two of the plurality of receiversbased on: at least one time difference between each of a plurality ofsignals received at the antenna elements connected to the receiver towhich the at least one circuit is coupled; and location data in at leastone of the plurality of signals.
 15. The system of claim 1, wherein thereceiver comprises a second input port and is configured to receivesignals from a single one of the plurality of receive antenna elementsvia the second input port.
 16. The system of claim 1, wherein at leastone of the plurality of receive antenna elements comprises a directionalantenna.
 17. The system of claim 1, wherein: the circuit is configuredto use the known, fixed delays in determining the time differences. 18.The system of claim 1, wherein the time of arrival estimator isconfigured to use the known, fixed delays to associate each individualreceived signal with the one of the plurality of receive antennaelements that receives the signal.
 19. The system of claim 1, whereinthe receiver de-spreads the signals.
 20. The system of claim 1, whereinthe circuit is further configured to perform peak detection on thesignals or leading edge detection on the signals.
 21. The system ofclaim 1, wherein the angle of arrival estimator is further configured togenerate a history of determined angular orientations.
 22. The system ofclaim 1, wherein the receiver comprises a RF interrogator.
 23. Thesystem of claim 22, wherein the RF interrogator is configured togenerate an interrogation and transmit the interrogation; the receiveris configured to observe a time of arrival of a signal sent in responseto the interrogation; and the time of arrival estimator is configured toobtain an estimate of a range from the receiver to a source of thesignal sent in response to the interrogation based upon the timedifference between the transmission of the interrogation and thereception of the signal from the source.
 24. The system of claim 1,wherein the receiver comprises a GPS receiver, and the signals compriseGPS signals.
 25. The system of claim 1, wherein the receiver comprises anavigation receiver, and the signals comprise navigation signals. 26.The system of claim 1, wherein the time of arrival estimator is furtherconfigured to perform re-sampling on the signals when determining thetime differences.
 27. A directional antenna system comprising: atransmitter comprising at least a single output port; a plurality oftransmit antenna elements fixedly configured into a known geometricrelationship to each other, the transmit antenna elements each beingcoupled to the single output port by one of a plurality of circuits,each of the plurality of circuits adding one of a plurality of known,fixed delays; a receiver, comprising at least one receive antennaelement and an output, configured to receive signals from the transmitantenna elements from the single output port of the transmitter; and acircuit, coupled to the output of the receiver, including a time ofarrival estimator configured to determine, from the signals receivedfrom the single output port of the transmitter, time differences atwhich signals from the transmitter are incident upon the antennaelements and an angle of arrival estimator configured to determine anangular orientation of the transmit antenna elements to the at least onereceive antenna element based on the time differences and the known,fixed delays.
 28. The system of claim 27, wherein the receiver ismounted on a vehicle.
 29. The system of claim 28, wherein the vehicle isan aircraft.
 30. The system of claim 29, wherein the angle of arrivalestimator is further configured to receive a calibration signalindicating a change in the known geometric relationship and compensatebased on the change when determining the time differences.
 31. Thesystem of claim 27, wherein: the receiver is one of a plurality ofreceivers, each of the plurality of receivers comprising at least onereceive antenna element and a single output; the circuit is one of aplurality of circuits, each circuit being coupled to the single outputof one of the plurality of receivers, each circuit including the time ofarrival estimator and the angle of arrival estimator; and the time ofarrival estimator and the angle of arrival estimator of at least one ofthe plurality of circuits are configured to determine a distance betweenat least two of the plurality of receivers based on: at least one timedifference between each of a plurality of signals received at the atleast one antenna element connected to the receiver to which the atleast one circuit is coupled; and location data in at least one of theplurality of signals.
 32. The system of claim 27, wherein the angle ofarrival estimator is configured to determine a location of the receiverrelative to the transmitter based on the angular orientation.
 33. Thesystem of claim 27, wherein: the receiver comprises a RF interrogator;and the system further comprises a second receiver associated with thetransmitter.
 34. The system of claim 33, wherein: the RF interrogator isconfigured to generate an interrogation and transmit the interrogation;the second receiver is configured to receive the interrogation and timea transmission of the interrogation with at least one of the known,fixed delays from the reception of the interrogation; the receiver isconfigured to observe a time of arrival of the signal from the transmitantenna elements; and the circuit is configured to obtain an estimate ofa range from the receiver to the transmitter based upon the timedifference between the transmission of the interrogation and thereception of the signal from the transmit antenna elements, wherein theestimate includes a compensation for the known, fixed delay.
 35. Adirectional receiver, comprising: a processor element comprising atleast a single input port; and a plurality of receive antenna elements,fixedly configured into a known geometric relationship to each other,each of the antenna elements being electrically connected to the singleinput port of the processor element by one of a plurality of circuits,each of the plurality of circuits adding one of a plurality of known,fixed delays such that outputs from each of the receive antenna elementsare received at the single input port of the receiver; wherein a timeinterval from a time at which a signal is incident upon each of theantenna elements to a time of arrival of energy collected by eachantenna element at the single input port of the processor element isknown, and electrical connections of the receive antenna elements to thesingle input port of the processor element are so configured that asignal observed by the processor element is a time domain sum of thesignals from the plurality of receive antenna elements; and wherein theprocessor element is configured to: perform analog-to-digital samplingto generate digital signals reflecting waveforms present in the observedsignal, perform signal processing to apply signal conditioning andde-spreading processes to the digital signals to generate de-spreadsignal samples, calculate a signal to noise ratio (SNR) and complexamplitude of each de-spread signal sample, apply an algorithm forestimating time of arrival of signals of interest in the de-spreadsignal sample, and estimate a time difference of arrival of the observedsignals originating in pairs of the plurality of receive antennaelements.